. It won't require large blades to work, but, as frollard points out, your generator is probably capable of putting out quite a bit of power. More power out requires more power in - bigger blades. If you only want to run an LED, you won't need the generator's full output, so you won't need as much power in - you can use smaller blades. If you want to use all the power the generator will put out, it will, as L says, require fairly large (by most DIY standards) blades.